The very fact that Hillary has been the subject of such smears for so long (murder of Vince Foster anyone?) means that her image in the public mind is comparatively fixed. Hillary's ability to dent her "unfavorables" will be quite limited, for the clay has hardened. The large proportion of the public that, as polls show, believes that Hillary is not honest or trustworthy are unlikely to be persuaded otherwise.
By contrast, the fact that Bernie is still introducing himself to the broad swath of the American public gives him a potential advantage for the fall. Dishonest attacks can be countered: don't wait for them to take hold (don't stand by and be "swift-boated," as John Kerry did); and find creative ways to turn the issues raised to one's advantage and against one's opponent. If Bernie and his team can be adept at this counter-punching -- a big "if," admittedly -- all those millions of dollars of attacks need not do irreparable damage.
Plus, in Donald Trump, one has a target for attacks of one's own--attacks that are entirely truthful, and entirely relevant to the interests and values at stake for the American people in this election.
This is not a one-size fits all kind of task.
Donald Trump represents a unique set of challenges for any opponent -- as the Republican contest has so clearly demonstrated. A candidate well-equipped for a conventional political battle might not be adept at dealing with someone like Donald Trump--a bully whose proposals lack anything like programmatic substance, and who continually speaks without any regard for the truth.